The usefulness of polar ethylene interpolymers in coatings and adhesives applications is well known. Often, such polar ethylene interpolymers are employed in conjunction with a tackifier in order to promote adhesion to a wider variety of substrates. Such compositions are commonly employed as hot melt adhesives, or in solvent based systems which set by evaporation of the solvent.
In many instances, especially where a superior bond is desired with a polar substrate, it is desirable to use polymers in which ethylene is interpolymerized with a relatively high proportion of an acidic comonomer. However, the acid content of ethylene interpolymers has generally been limited in systems employing a tackifier because of compatibility problems with the tackifier. For example, in U.S. Pat. No. 3,869,416 there is described a reinforced petroleum wax composition useful as a hot melt adhesive which includes an adhesion promoting system consisting essentially of a polar ethylene interpolymer and a tackifier such as wood rosin or its derivatives. It is disclosed therein that when the polar ethylene interpolymer is ethylene interpolymerized with carboxylic acids, the interpolymerized acid moieties should not exceed 5 mole percent to insure that there is at least partial solubility of the interpolymer in the other ingredients. In addition, the useful ranges of proportions of the interpolymer and tackifier described is extremely limited.
In aqueous adhesive systems containing both a polar ethylene interpolymer and a tackifier, the proportion of acid comonomer in the ethylene interpolymer has also been limited. For example, in U.S. Pat. No. 3,487,036 it was preferred to use ethylene terpolymers containing 0.5-10 percent by weight acrylic acid or methacrylic acid and having a combined vinyl acetate and acid content of 18-40 percent by weight. Similarly, in the dispersion described in U.S. Pat. No. 3,823,108, there was employed an ethylene terpolymer consisting essentially of about 55-80 weight percent ethylene, about 10-42 weight percent ester monomer, and about 3-10 weight percent .alpha.,.beta.-ethylenically unsaturated carboxylic acid. Also, in U.S. Pat. No. 4,289,669, there was described an aqueous dispersion containing an alkenyl succinic anhydride, a tackifier, and an ethylene-acrylic acid or -methacrylic acid interpolymer which had an acid number within a range of about 0.25-30.
The only heretofore known solvent based adhesive system containing both a tackifier and an ethylene interpolymer having interpolymerized therein an ethylenically unsaturated carboxylic acid in a proportion greater than 12 weight percent of which the applicants are aware is described in U.S. Pat. No. 3,853,606. However, this adhesive system is not a true aqueous dispersion, but is instead a solvent-in-water emulsion which is prepared by dispersing an ethylene interpolymer containing 18-24 weight percent of interpolymerized acrylic acid in an aqueous solution of a volatile alkali, the alkali being driven off by heating, and emulsifying into the interpolymer solution a tackifier which has previously been dissolved in an organic solvent. Thus, there are two separate and distinct phases, a continuous aqueous phase containing the ethylene interpolymer, and a dispersed organic solvent phase containing the dissolved tackifier. Such a system is generally disadvantageous because of the handling problems posed by the solvent during preparation and drying of the composition, including, for example, the flammability and/or toxicity of the solvent. Moreover, the dried or set coating resulting from such an adhesive system is characterized as an emulsion of tackifier particles and ethylene interpolymer particles, rather than a homogeneous, single-phase coating. These particles can separate into different phases, especially when exposed to heat.
Other important characteristics of an aqueous based adhesive system are the viscosity and the solids content. Generally, adhesives or coatings solutions with a viscosity greater than about 1500 Pa-s (1500 cp) are not commercially attractive because of the difficulty in applying them to a substrate. While it is desirable to have as high a solids content as is practical because of the reduced time and energy required for drying or setting, it is generally necessary to limit the solids content of the system to obtain a suitable viscosity. While it is possible to heat the adhesive system to reduce the viscosity, this requires specially modified equipment for applying the solution, and is not acceptable with heat-sensitive substrates.
The heretofore known solvent based adhesive systems containing a polar ethylene interpolymer and a tackifier have had serious shortcomings with respect to viscosity and solids content. For example, in U.S. Pat. No. 3,823,108 discussed above, there was reported a viscosity of 3100 Pa-s at only 20 percent solids.
When polymers of ethylene interpolymerized with an ethylenically unsaturated carboxylic acid are employed in an aqueous adhesive system, it is also important to consider the amount and type of alkali required to disperse the interpolymer. For example, in U.S. Pat. No. 3,823,108 there was required an alkali metal hydroxide in an amount sufficient to neutralize 10-100 percent of the acid groups in the ethylene interpolymer and the tackifier, and an amine base in an amount sufficient to neutralize 0-300 percent of the acid groups in the ethylene interpolymer and the tackifier, the total amount of alkali metal hydroxide and amine base being sufficient to neutralize at least about 50 percent of the acid groups. The presence of a strong base such as an alkali metal hydroxide is not desirable because of the corrosion which results to processing equipment and to the substrate. In addition, since such bases are non-volatile and are not removed upon drying, the ethylene interpolymer remains in a cross-linked form which may result in a reduction in adhesion to polar substrates. With the exception of very dilute dispersions, it has not heretofore been known to prepare, without the aid of substantial quantities of a strong alkali or a water-immiscible solvent for the tackifier, a dispersion containing both a highly acidic ethylene interpolymer and a tackifier.
Several methods are known for preparing solvent based adhesive systems containing polar ethylene interpolymers and tackifiers. One type of method involves the use of a water-immiscible solvent to dissolve the tackifier, such as in U.S. Pat. No. 3,853,606, or both the interpolymer and the tackifier, such as in U.S. Pat. No. 3,487,036. In this method, the solvent containing the tackifier and/or interpolymer is emulsified in water or in aqueous interpolymer solution. The resulting emulsion is then used as the adhesive system and applied to a substrate, as in U.S. Pat. No. 3,853,606, or alternatively, the solvent is first evaporated from the emulsion as in U.S. Pat. No. 3,487,036. This type of method is not desirable because of the handling problems concomitant with the use of a solvent other than water.
The other method involves the direct dispersion of the polar ethylene interpolymer and the tackifier into an aqueous medium, as described in U.S. Pat. No. 3,823,108 and U.S. Pat. No. 4,289,669. A major drawback with this type of method has been the low conversion rate of dispersing the interpolymer and tackifier into the aqueous medium. For example, in U.S. Pat. No. 3,823,108 the conversion of the ethylene terpolymer to hydrosol was reported to range from 60-98 percent. Further, the undispersed tackifier and interpolymer must typically be removed by filtration before the dispersion can be used.